powerpc/mpc8349: Use generic mpc85xx DDR driver

MPC8349 has been using mpc85xx DDR driver through a symbolic link to
mpc85xx_ddr_gen2.c. After consolidating the drivers to a single set
under driver/ddr/fsl/, the link is replaced by referring driver
directly. We now can simply enable the macro and use the driver.
Other mpc83xx SoCs still use their own driver.

Signed-off-by: York Sun <yorksun@freescale.com>